Teaching the discipline of requirements engineering (RE) is slowly establishing at universities within the software engineering curriculum. While several studies have shown that case study-based education was more efficient in RE, many teachers are still reluctant to change their teaching style, and stay with classical lectures and complementary exercises. These courses often fail to relate the different steps and stages of RE to each other and do not address crucial communication and project management issues that are common in industrial RE practice. They also miss the chance for using the classroom as a near-to-real-settings research lab, and won't show students the stakes existing in doing engineering in our society. We describe our experiences in teaching RE with a case study in two universities, achieving a triple-win: putting students in contact with real stakeholders, showing students their responsibility towards a sustainable world and doing empirical research in the classroom. We report on the course design, the evaluation, the lessons learned, and the potential success factors for such courses. We conclude that case study-based approaches to teaching RE considerably improve skills valued by industry, are feasible at a reasonable cost, and are enjoyable for the students, the teachers and the stakeholders. With this paper, we want to encourage RE educators to implement such courses in their setting.